Managers frequently assume that workers who do not display observable symptoms of physical illness are simply wanting a rest from work when they lodge claims for sick leave. However, a number of ailments do not manifest apparent symptoms. Some of these hidden conditions, such as mental disorders, severe stress and emotional disturbance, and alcohol and drug abuse are perceived to have a stigma attached to them, so that workers may choose to be uncommunicative about their illness, or else invent a more 'acceptable' ailment, which over time often raises the suspicion of the manager. In such cases, managers may believe such workers to be indolent and unreliable, when in fact the irony is that they are genuinely unwell. This paper examines the incidence of a number of so-called 'hidden' ailments, showing how widespread they are, and how managers are likely to misjudge a significant amount of their absenteeism as voluntary, when in fact the level of legitimate illness necessitating sick leave may well be higher than believed.